In the theater-of-operations, warfighters encounter threats that contain RF electronics. Such RF electronic devices are generally small (man-portable) and typically buried or located close to the ground, making it difficult to distinguish from background clutter using traditional linear radar.
Whereas linear radar exploits the reflection from a target whose frequencies are the same as those transmitted, nonlinear radar exploits the electronic response from a target whose reflected frequencies are different from those transmitted. Reception of frequencies that are not part of the transmitted probe distinguishes the received signal from a linear return that can be produced by clutter and indicates the presence of an electronic circuit. For the warfighter, the presence of an electronic circuit (in a location that typically does not contain an electronic circuit) implies the presence of a threat. Ultra-wideband (UWB) ground-penetrating radar (GPR) is a linear radar technology for detecting concealed targets such as landmines and other explosive devices. UWB GPR attempts to detect a threat set similar to that of the present invention. Since electronics and clutter both produce linear radar reflect ions, UWB GPR systems require a greater degree of signal processing to separate targets from clutter. By confining the detectable target response to nonlinear interactions, nonlinear radar is able to more easily separate targets from clutter.
Nonlinear radar is capable of detecting almost any un-shielded electronics, whether the electronics are on or off. Nonlinear radar exploits the electronic response from a target whose reflected frequencies are different from those transmitted. Reception of frequencies that are not part of the transmitted probe distinguishes the received signal from a linear return produced by clutter and indicates the presence of electronics. Several devices and methods exist for identifying electronics and other manmade objects using the nonlinear responses of metal and semiconductor junctions. Some detectors tune to the harmonics of a single-frequency radar transmission, such as in U.S. Pat. No. 3,732,567 to Low. Other detectors tune to the intermodulation produced by the interaction of multiple frequencies at the target, such as discussed in “A practical superheterodyne-receiver detector using stimulated emissions,” by C. Stagner, et al., in IEEE Trans. Instrum. Meas., vol. 60, no. 4, pp. 1461-1468 (April 2011) (herein incorporated by reference). In the Stagner, et al. paper, the unintended emissions of super heterodyne receivers are analyzed for the detection of radio-controlled explosives. Arbitrary signals are injected into a radio's unintended emissions using a relatively weak stimulation signal, referred to as stimulated emissions. Intermodulation products are generally the result of odd-order nonlinear interactions.